The invention relates to a method designed to be practiced in a circulation lubrication system comprising a lubricating oil tank; pressure pipe lines for supplying oil to parts to be lubricated; return pipe lines for returning the oil from the parts to be lubricated to the oil tank; means for pumping oil into the pressure pipe lines; and adjusting means for maintaining a desired lubrication situation, wherein air is blown into the lubricating oil tank. The invention further relates to an arrangement for blowing air into the oil tank in a circulation lubrication system.
Circulation lubrication systems are today used widely to lubricate various machines especially when the lubricant is used for cooling the part needing lubrication. An example of the use of circulation lubrication is the lubrication of the bearings of the drying cylinders in paper machines or the lubrication of gas turbines. In both cases the bearings are subject to a thermal load from the outside.
Circulation lubrication is also used when the part to be lubricated creates considerable dissipation power. One example of such applications is the lubrication of gear boxes.
Still another use of circulation lubrication is in cases where the lubricant may get dirty in the part to be lubricated, and it should be possible to reuse the lubricant after cleaning.
A circulation lubrication system usually comprises a pump operated by an electric motor. The pump brings the oil into motion. The output of the pump is usually selected so that it exceeds the required flow rate by 10 to 20%, so that a sufficient operating margin is left for the control of bypass pressure. Oil is arranged to pass through replaceable filters. The filters are often arranged in two groups so that one group can be separated by means of Valves and the filters changed without having to stop the entire assembly. An electric or steam-operated heater is often used for further heating the oil. The oil is cooled by a heat exchanger having water or air cooling means. The cooling power is adjusted by a temperate regulator provided for supply oil. The set value of the temperature regulator is often typically about 55.degree. C. Pressure control is often effected by a bypass valve back to the oil tank. Depending on the system, the set values of the pressure controller typically vary between 5 and 20 bar.
Oil is supplied to parts needing lubrication, such as different parts in a paper machine, by means of pressure trunk pipes usually made of a rustproof material. The oil flows in the pipes as a laminar flow, and so the pressure drop is small. From the trunk pipes, the oil is distributed to a plurality of flow-metering boards wherefrom it is divided into rising pipes, to be supplied to a number of parts to be lubricated. From these parts, the oil is passed through return pipes by utilizing gravitation, into collector pipes on the return side. It is passed, under the influence of gravitation, through the collector pipes, into the tank of the circulation lubrication assembly. The pipes on the return side are never full of oil. The inclination of the pipes is about 2 to 3% towards the tank of the assembly. Before the tank, the return oil is passed through a coarse-mesh filter. Oil returned into the tank is then again sucked into circulation by the pump. The dimensions of the tank are usually such that the oil stays in the tank for 10 to 50 minutes, depending on the viscosity of the oil.
In the system, the tank has several functions for conditioning the oil before recirculation. One function of the tank is to allow air bubbles contained in the oil to rise to the surface. Air bubbles may increase the liability of the pump to cavitation, and they may deteriorate the lubricant film formed on the part needing lubrication. Another function is to allow large dirt particles having a density considerably higher than that of oil to deposit on the bottom of the tank. To some extent, water drops contained in the oil will also fall down in the tank. The density of water is so close to that of oil that the falling speed is low. The tank condenses moist air flowing in the return pipes on the tank walls and drains the water along the walls, thus collecting it on the bottom of the tank. The tank also cools the oil through the walls. Furthermore, the tank forms a space into which the pipe lines are emptied at system shut-downs, and the tank serves as an oil storage vessel in the case of a pipe leakage, so that the machine to be lubricated can be shut down controllably. Still another function of the tank is to ensure that there is always oil in the pump suction means, in order that air will not be sucked along. The moisture and suitable temperature also enable bacteria to grow in the tank.
The tank also has associated therewith thermostat-controlled oil heaters, which may be electric heaters or steam-operated heaters. The heaters are used to heat the great amount of oil contained in the oil tank within 4 to 8 hours, typically to about 55.degree. C., before the system is started, as the screw pumps used are not able to pump cold oil of high viscosity at full power without the risk of cavitation.
In principle, the systems described above operate well; in practice, however, they have several drawbacks, as a result of which the operation of the systems is not the best possible. One drawback is the entrainment of moist air into the tank through the return pipes, as the oil sucks air along with it from the part to be lubricated, and the air carries dirt and moisture with it. This drawback is especially apparent in paper machines, as there is plenty of both dirt and moisture below the hood, in the drying section. Moreover, it is to be noted that the temperature is high below the hood due to the steam heating of the drying cylinders. The moisture content of the air is high due to the moisture evaporated from the paper web. As a result of the flow of moist air, considerable amounts of water are condensed in the tank, which causes problems in the tank.